1. Field of the Invention
The present invention is generally related to a method for controlling the tractive force of a ground vehicle and, more particularly, to a method for controlling the degree of slip between a driven wheel or tread and the ground.
2. Description of the Prior Art
Many different types of traction control systems are known to those skilled in the art. U.S. Pat. No. 6,154,702, which issued to Fodor et al on Nov. 28, 2000, describes a method and apparatus for estimating applied wheel torque in a motor vehicle. The vehicle traction control system is controlled in part by a signal value indicative of estimated wheel torque. The estimated wheel torque value is produced within the vehicle""s electronic engine control module by summing a first value which indicated the estimated torque attributable to engine combustion and a second value which is proportional to engine acceleration/deceleration which indicates the amount of torque attributable to the inertial movement of engine and drive train masses.
U.S. Pat. No. 4,598,611, which issued to Frank on Jul. 8, 1986, describes a low power control system and method for a power delivery system having a continuously variable ratio transmission. The control system and method for a power delivery system, such as that of an automotive vehicle, has an engine coupled to a continuously variable ratio transmission. Independent control of engine and transmission enable the engine to precisely follow a desired operating characteristic, such as the ideal operating line for low fuel consumption. Engine fuel requirements, such as throttle position, are a function of at least measured engine speed, and are adjusted in accordance with any load placed on the engine so that, during low power operation, fuel flow is increased when a decrease in operating speed occurs, and fuel flow is reduced when an increase in operating speed occurs.
U.S. Pat. No. 6,125,314, which issued to Graf et al on Sep. 26, 2000, describes a drive train controller for a motor vehicle. The controller is used to calculate the position of the accelerator pedal which is interpreted as the wheel torque or transmission output torque desired by the driver, in order to calculate setpoint values for the torque to be output by the drive train. It contains a control circuit in which the desired wheel torque is evaluated, together with further operating parameters of the motor vehicle, in a fuzzy system. It outputs an output signal by which the wheel torque to be exerted on the roadway by the wheels of a motor vehicle is defined.
U.S. Pat. No. 6,061,622, which issued to Probst on May 9, 2000, describes a circuit configuration for controlling torque transmitted between driven wheels of a motor vehicle and a roadway. The circuit configuration for controlling engine torque transmitted to a roadway through driven wheels of a motor vehicle includes a device for wheel-selective slop value detection and for determining a representative wheel slip, a device for specifying a set point slip, and an engine controller that takes a slip status into account. The engine controller is preceded by a regulator, to which a difference between the set point slip and the representative wheel slip is supplied as a control difference. A feedback signal characterizing the current engine torque is sent from the engine controller to the regulator.
U.S. Pat. No. 6,002,979, which issued to Ishizu on Dec. 14, 1999, describes a traction control system for automotive vehicles. The system includes a traction control unit for calculating a slip amount as the difference between a drive wheel speed and a target drive wheel speed, an engine control unit cooperating with a fuel supply system for decreasingly compensating the engine power by the fuel out control action in response to the slip amount, and a temperature sensor for detecting an internal combustion engine temperature. The traction control unit is responsive to the internal combustion engine temperature for properly setting the target drive wheel speed or for properly setting control gains of the engine control system so that the engine speed is maintained above a minimum internal combustion engine revolution speed at which engine stall is prevented.
U.S. Pat. No. 5,737,713, which issued to Ikeda et al on Apr. 7, 1998, describes a traction control system for a vehicle. An initial engine torque for a traction control is selected from the following engine torques: an engine torque calculated from a throttle opening degree and an engine revolution number; a required engine torque calculated from a total acceleration and a vehicle speed; and an engine torque corresponding to a road surface of an extremely low friction coefficient, depending upon a slipping state determined by comparing a driven wheel speed with slip reference values.
U.S. Pat. No. 5,463,551, which issued to Milunas on Oct. 31, 1995, describes an integrated traction control system. The approach provides for smooth and precise control of engine output torque to reduce slip of the driven wheels during acceleration thereof through an integrated control of engine ignition timing and engine fueling in automotive vehicles regardless of the type of transmission used in the vehicle. In the presence of a slip condition at the vehicle driven wheels, ignition timing is adjusted to attempt to alleviate the condition.
U.S. patent application Ser. No. 09/452,797 which was filed by Suhre on Dec. 2, 1999 and is now abandoned, discloses a propeller pitch selection method for a controllable pitch propeller. A propeller blade pitch selection method receives input signals from a manually controlled input, such as a throttle handle, and from a device that measures the velocity of a marine vessel. This can be provided by a speedometer or a GPS system. The vessel velocity can be measured relative to the water in which the vessel is operated or, alternatively, relative to a fixed position on earth. Using the two inputs of thrust demand from the operator and vessel velocity from the speedometer, the engine control unit selects both a blade pitch angle and an engine torque demand magnitude from stored data and uses the two selective values to set the torque demand of the engine and the blade pitch position of the propeller. An adaptive system is also provided in which steady state conditions allow the ECU, or engine control unit, to make slight incremental changes in blade pitch when the marine vessel is operating under constant thrust conditions so that the ECU can determine if the blade pitch settings are optimal. If an improvement can be achieved by modifying the preselected blade pitch settings, an adaptive matrix is used to add corrective values to the preselected pitch magnitudes.
U.S. Pat. No. 6,298,614 which was filed by Suhre on Feb. 9, 2001 discloses an engine control system using an air and fuel control strategy based on torque demand. A control system for a fuel injected engine provides an engine control unit that receives signals from a throttle handle that is manually manipulated by an operator of a marine vessel. The engine control unit also measures engine speed and various other parameters, such as manifold absolute pressure, temperature, barometric pressure, and throttle position. The engine control unit controls the timing of fuel injectors and the injection system and also controls the position a throttle plate. No direct connection is provided between a manually manipulated throttle handle and a throttle plate. All operating parameters are either calculated as a function of ambient conditions or determined by selecting parameters from matrices which allow the engine control unit to set the operating parameters as a function of engine speed and torque demand, as represented by the position of throttle handle.
The patents and patent applications described above are hereby explicitly incorporated by reference in the description of the preferred embodiment of the present invention.
A method of controlling tractive force of a ground vehicle, in accordance with a preferred embodiment of the present invention, comprises the steps of receiving a tractive force request and determining a tractive force command as a fractional portion of the tractive force request. The tractive force request is typically received from a manually controlled foot pedal or a manually controlled hand grip which an operator of the ground vehicle manipulates to provide a tractive force request to an engine control unit. As a function of the tractive force request, the present invention determines a tractive force command which is a fractional portion of the tractive force request. The fractional portion, which can be equal to unity, is determined as a function of certain operational parameters relating to the operation of the ground vehicle, such as magnitude of slip of its driven axle.
A preferred embodiment of the present invention further comprises the steps of determining a rotational velocity of a shaft, or axle, selected from the group consisting of a driven shaft connected in torque transmitting relation with a source of motive power and a non driven shaft which is not connected in torque transmitting relation with said source of motive power. The source of motive power is typically an internal combustion engine or an electric motor. The driven shaft is connected to the engine in order to provide torque to a driven wheel or a driven traction device which is in contact with the ground and provides the thrust for the ground vehicle. The non driven shaft assists in supporting the ground vehicle but does not transmit torque to a wheel or other component. As an example, a two wheel drive automobile typically has two driven wheels which are driven by two driven shafts which are, in turn, driven by an internal combustion engine and transmission. The other two wheels of a two wheel drive vehicle are non drive wheels. The present invention is applicable, in alternative forms, to ground vehicles that have either only driven wheels or a combination of driven and non driven wheels. Throughout the description of the present invention, it should be understood that the driven shafts and non driven shafts are typically connected to wheels in certain wheeled ground vehicles and the driven shafts are typically connected to some type of tractive device, such as a rubber tread, in ground vehicles such as snowmobiles.
A preferred embodiment of the present invention further comprises the steps of selecting a desired gear ratio between the source of motive power and the driven shaft. The desired gear ratio is selected as a function of the tractive force command and the rotational velocity. It should be understood that the present invention is most appropriately used in conjunction with a continuously variable transmission (CVT), but can be used with alternative types of transmissions. It should also be understood that the fractional portion of the tractive force request used to determine the tractive force command is typically equal to 1.00 or less. In other words, under normal conditions when no slip occurs between the driven wheel and the ground, the fractional portion would be generally equal to approximately unity. However, if measurable slip occurs, the fractional portion can be decreased so that the tractive force command is significantly less than the tractive force request provided by the operator of the ground vehicle. The desired gear ratio selected by the present invention represents the gear ratio between one rotational component attached to a driveshaft of the engine and another rotational component attached to the driven shaft of the ground vehicle.
A preferred embodiment of the present invention further comprises the steps of selecting an optimal brake torque magnitude as a function of the tractive force command and a rotational velocity, causing a gear ratio between the source of motive power and the driven shaft of the ground vehicle to be generally equal to the desired gear ratio, causing the engine to provide a brake torque output that is generally equal to the optimal brake torque magnitude, and determining a magnitude of slip between the ground and a driven member attached in torque transmitting relation with the driven shaft. Throughout the description of the present invention, the term xe2x80x9cbrake torquexe2x80x9d shall mean the indicated torque minus both the inertia of the powertrain multiplied by the angular acceleration and the frictional effects. Therefore, when a torque command is issued to the engine, it is generally equal to the brake torque command plus the frictional effect and the powertrain inertia multiplied by the angular acceleration. In other words, the brake torque is the magnitude of torque actually delivered to the axles or treads while the indicated torque is the theoretical torque provided by the engine if no frictional effects or inertia effects existed.
In addition, the present invention comprises the step of changing the fractional portion as a function of the magnitude of slip. In different embodiments of the present invention, the fractional portion is changed as a function of the magnitude of slip in different ways to accommodate the different characteristics of the ground vehicle. In practice, it is likely that the actual magnitude of the fractional portion will not actually be calculated. Instead, it is most likely that an adjusted magnitude of the tractive force command will be determined as a function of certain monitored operating parameters and, as a result, the adjusted tractive force command will be equal to some fractional portion of the tractive force request. The ratio of the tractive force command to the tractive force request is the resulting fractional portion magnitude.
The rotational velocity determined by the present invention can be the rotational velocity of the driven shaft or non driven shaft, depending on the type of ground vehicle used in conjunction with the present invention. If the rotational velocity is that of the driven shaft, such as in a snowmobile application or a four wheel drive application, the present invention further comprises the steps of measuring elapsed time and determining a rotational acceleration of the driven shaft as a function of the rotational velocity of the driven shaft and the elapsed time. In other words, knowing the velocity at various instance of time allows the system to calculate the change in velocity, or acceleration, during a preselected period. The present invention comprises the steps of selecting an upper acceleration limit magnitude as a function of the tractive force command and the rotational velocity of the driven shaft and comparing the rotational acceleration of the driven shaft to the upper acceleration limit magnitude. The present invention then changes the fractional portion as a function of the relative magnitudes of the rotational acceleration of the driven shaft and the upper acceleration limit magnitude. In a preferred embodiment of the present invention, the desired gear ratio, the optimum brake torque magnitude, the upper acceleration limit magnitude, and the upper slip limit magnitude are selected from two dimensional arrays in which values for these parameters are arranged as a function of the tractive force command and the rotational velocity of either the driven shaft or the non driven shaft.
The rotational velocity determining step can comprise the individual steps of determining a rotational velocity of the driven shaft and determining the rotational velocity of the non driven shaft. An upper slip limit magnitude is selected as a function of the tractive force command and the rotational velocity of the non driven shaft and a slip value is calculated as a function of the rotational velocity of the driven and non driven shafts. The present invention then compares the slip value to the upper slip limit magnitude and changes the fractional portion as a function of the relative magnitudes of the slip value to the upper slip limit magnitude.